Method of making pulp having high hemicellulose content

ABSTRACT

Bleached, Kraft chemical pulp having increased hemicellulose content compared to conventional Kraft chemical pulps. In one embodiment, the pulp has a hemicellulose content greater than about 17 weight percent as measured by the 18% caustic solubility test. In another aspect, a method for making wood cellulose pulp having increased hemicellulose content compared to conventional Kraft chemical pulps. In the method, a lignocellulosic material is pulped in caustic sulfide to provide a first brownstock, which is treated with oxygen to provide a second brownstock, which is then bleached to provide the pulp product.

FIELD OF THE INVENTION

[0001] The present invention relates to cellulose pulp and, moreparticularly, to chemical pulp having increased hemicellulose content.

BACKGROUND OF THE INVENTION

[0002] Pulp refers to cellulose fibers that have been liberated fromwood or other lignocellulosic materials through physical and/or chemicalprocesses. Pulp manufactured for papermaking applications is generallydispersed in water as a slurry and then reformed into a web or sheet.

[0003] Pulping methods include chemical, mechanical, andchemi-mechanical processes. While mechanical pulping methods rely onphysical action and thermal energy to liberate fibers, chemical pulpingmethods rely on the effect of chemicals to separate fibers from rawmaterials. Generally, the more chemicals used in a pulping process, thelower the pulp yield and lignin content because chemical action degradesthe wood and solubilizes wood components including lignin andhemicellulose. While mechanical pulp yields are typically greater thanabout 90 percent, chemical pulp yields are considerably lower, moreoften in the range from about 40 to about 65 percent.

[0004] Although chemical pulping methods result in lower pulp yields,these methods provide individual pulp fibers that are uncut, which is incontrast to fibers produced from mechanical pulping methods.Furthermore, pulp fibers produced by chemical pulping methods providepaper products having increased strength because lignin, whichinterferes with interfiber hydrogen bonding, is largely removed duringthe process. Of the chemical pulping processes, the Kraft processproduces generally stronger pulps compared with the other major chemicalpulping method, the sulfite process.

[0005] Chemical pulping of wood and other lignocellulosic materialsresults in delignification of the raw material. Delignification is theprocess of breaking down of lignin and rendering the breakdown productssoluble in the cooking liquor (alkali), such that the lignin can beremoved from the cellulose. A pulp's lignin content can be measured bykappa number. The higher the measured kappa number, the greater a pulp'slignin content. The kappa number is often used to monitordelignification during and after pulping processes. A similar test isthe permanganate number.

[0006] Pulps produced by the Kraft pulping process have a light to darkbrown color and, as a consequence, are bleached to increase the pulp'sbrightness.

[0007] Accordingly, there exists a need for a Kraft pulp that offers theadvantage of strength associated with conventional Kraft pulps, while atthe same time has improved yield and increased hemicellulose contentcompared to conventional Kraft chemical pulps. The present inventionseeks to fulfill these needs and provides further related advantages.

SUMMARY OF THE INVENTION

[0008] In one aspect of the invention, a Kraft chemical pulp havingincreased hemicellulose content is provided. The Kraft chemical pulp isa wood pulp having increased hemicellulose content compared toconventional Kraft chemical pulps. In one embodiment, the pulp has ahemicellulose content greater than about 17 weight percent as measuredby the 18% caustic solubility test. The pulp can be incorporated intosheets having initial (unrefined) tensile strengths significantlygreater than sheets incorporating conventional Kraft chemical pulpfibers. The pulp is more readily refined than conventional chemicalpulps and achieves strength/drainage properties at significantly lessrefining energy. The pulp can be produced in higher yield than otherKraft chemical pulps.

[0009] In another aspect, the invention provides a method for makingcellulose pulp having increased hemicellulose content. In the method, alignocellulosic material is pulped with a liquor, which includes sodiumhydroxide (NaOH), sodium sulfide (Na₂S), anthraquinone, and someresidual sodium carbonate (Na₂CO₃) from the causticizing process, toprovide a first brownstock. The first brownstock is treated with oxygento provide a second brownstock, which is then bleached to provide thepulp product. The caustic sulfide includes polysulfide andanthraquinone. Chlorine dioxide is used in the bleaching.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomebetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0011]FIG. 1 is a flow diagram illustrating of a representative methodfor producing the pulp of the invention; and

[0012]FIG. 2 is a graph comparing percent yield as a function of kappanumber for pulps made by the method of the invention (A) and for pulpsmade by conventional cooking (B), digester yields for the methods arealso illustrated (C and D, respectively).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0013] In one aspect, the present invention provides a wood cellulosepulp having increased hemicellulose content. The Kraft chemical pulp ofthe invention is a wood pulp having increased hemicellulose contentcompared to conventional Kraft chemical pulps. Because hemicelluloseimparts flexibility to a fiber, the pulp fibers of the invention aremore flexible in papermaking systems and exhibit greater interfiberbonding compared to conventional Kraft chemical pulps. The increasedflexibility and interfiber bonding of the fibers renders sheets or websthat incorporate these fibers stronger than sheets or webs incorporatingconventional Kraft chemical pulp fibers. The pulp of the inventionprovides sheets having initial (unrefined) tensile strengthssignificantly greater than sheets incorporating conventional Kraftchemical pulp fibers.

[0014] As a result of the pulp fiber's flexibility and increasedhydrogen bonding potential, the amount of refining required to achieve aparticular sheet strength (tensile strength) or advantageous drainageproperty (Canadian Standard Freeness, CSF) is greatly reduced. Thus, thepulp of the invention can be refined to a predetermined point morereadily than a conventional Kraft chemical pulp making the pulp of theinvention more attractive from a refining energy requirement basis. Thepulp of the invention is more readily refined than conventional Kraftchemical pulps and achieves strength/drainage properties atsignificantly less refining energy.

[0015] Because the pulp of the invention has increased hemicellulosecontent compared to other Kraft chemical pulps, the pulp is produced inhigher yield than other Kraft chemical pulps.

[0016] The pulp of the invention is a fully bleached pulp suitable forpaper grade market pulp.

[0017] Hemicellulose Content. As noted above, the pulp of the inventionadvantageously has an increased hemicellulose content compared toconventional chemical pulps. In one embodiment, the pulp of theinvention has a hemicellulose content greater than about 17 percent asmeasured by the 18% caustic solubility test described below. Thehemicellulose content of the pulp of the invention is about 2 percentgreater than conventional chemical pulps. Other conventionally producedsoftwood Kraft pulps have a hemicellulose content less than about 16percent as measured by the 18% caustic solubility test.

[0018] The hemicellulose content of pulp can be measured by severalmethods. One empirical method is the 18% caustic solubility method(TAPPI T-235 CM-00). In this method, a weighed quantity of pulp (1.5 g)is soaked in 18 percent by weight aqueous sodium hydroxide (100 mL) for1 hour. During the soak, the pulp fibers swell and the pulp'shemicellulose dissolves into solution. The pulp is then filtered, and 10mL of the filtrate is mixed with 10 mL of potassium dichromate and 30 mLsulfuric acid. This solution is titrated with ferrous ammonium sulfate.The percent alkali solubility is then calculated using the amounts ofthe various solutions and the amount of pulp. The method is usually anunderestimate of hemicellulose content because not all of thehemicellulose is dissolved and removed from the pulp during thisprocedure. Hemicellulose content for a pulp may also be determined bysugar analysis of completely digested pulp. Such a determination wouldgenerally provide a higher hemicellulose content value than the 18%caustic solubility method.

[0019] Sheet Tensile Strength. The pulp of the invention has anunrefined sheet tensile strength significantly greater than other Kraftchemical pulps. Conventional chemical pulps provide sheets havinginitial tensile breaking lengths from about 2.5 to about 4.0 km asmeasured by the TAPPI T 220. The pulp of the invention provides sheetshaving initial tensile strengths of from about 4.5 to about 5.5. In oneembodiment, the pulp provides a sheet having an initial tensile strengthof about 5, which is at least a 20 percent increase compared toconventional Kraft chemical pulps.

[0020] Yield. Due at least in part to the pulp's increased hemicellulosecontent, the pulp of the invention is produced in greater yield thanother conventional Kraft chemical pulps. The yield provided by themethod of the invention is at least about 3.0 to about 3.5 percentgreater than for conventional Kraft pulps. FIG. 2 is a graph thatcompares percent yield as a function of kappa number for pulps made bythe method of the invention (e.g., cooked with polysulfide andanthraquinone and treated with oxygen) (Curve A in FIG. 2; from about 47to about 51 percent) and for pulps made by conventional cooking followedby oxygen treatment (Curve B in FIG. 2; from about 45 to about 47percent). Digester yields for these processes are also illustrated(Curves C and D, respectively, in FIG. 2). Yields are based on theweight of oven-dried chips introduced to the digester. For the pulpsmade by the method of the invention, the digester kappa was 33, 38, and43. For the pulps made by conventional methods, the digester kappa was34, 38, and 44.

[0021] Brightness and Lignin Content. The pulp of the invention is afully bleached pulp (i.e., brightness greater than 89% ISO). There aretwo principal types of measurements to determine the completeness of thepulping or bleaching process: the degree of delignification and thebrightness of the pulp.

[0022] There are many methods of measuring the degree of delignificationof the pulp. Most are variations of the permanganate test. The normalpermanganate test provides a permanganate or K number that is the numberof milliliters of 0.1 N potassium permanganate solution consumed by onegram of oven dry pulp under specified conditions (see TAPPI StandardTest T-214). The kappa number is similar to the permanganate number, butis measured under carefully controlled conditions and corrected to bethe equivalent of a 50 percent consumption of the permanganate solutionin contact with the sample. The test gives the degree of delignificationof pulps through a wider range of delignification than does thepermanganate number. The kappa test measures the consumption ofpermanganate ion by lignin. The kappa number is the number ofmilliliters of 0.1 M potassium permanganate consumed by one gram of pulpin 0.5 N sulfuric acid after a 10 minute reaction time at 25° C. underconditions such that one-half of the permanganate remains unreacted (seeTAPPI Standard Test T-236). The kappa number for the pulp of theinvention is described below in reference to the pulping process.

[0023] There are also a number of methods of measuring pulp brightness.Brightness is a measure of reflectivity and its value is expressed as apercent of some scale. Brightness can be measured by TAPPI Method T 525OM-92.

[0024] The brightness of the product pulp of the invention is in therange from about 75 to about 95% ISO. In one embodiment, the pulp'sbrightness is from about 88 to about 92% ISO. In another embodiment, thepulp has a brightness of about 90% ISO.

Pulping/Bleaching Methods

[0025] In another aspect of the invention, a method for making acellulose pulp having increased hemicellulose content pulp is provided.The method includes the steps of Kraft chemical pulping, oxygendelignification, and bleaching. The combination of steps in the methodprovides for delignification while maintaining hemicellulose content toimprove yield and provide a chemical pulp having the advantageouscharacteristics noted above.

[0026] A flow diagram illustrating a method for producing the pulp ofthe invention is illustrated in FIG. 1. Referring to FIG. 1, wood chipsupply 10 provides wood chips to digester 20. Liquor is provided to thedigester from white liquor supply 70 through polysulfide/anthraquinoneprocessor 80. The polysulfide/anthraquinone processor provides liquorcontaining polysulfide and anthraquinone to the digester. White liquorand brownstock from the digester are provided to oxygen reactors 30. Theoxygen treated pulp from the oxygen reactors is then provided to bleachstages 40. The bleached pulp from the bleaching stage is then provide topulp machine 50. At the pulp machine, the pulp is deposited onto aforaminous support (e.g., Fourdrinier), water is withdrawn, the wet pulppressed, and then dried to provided the pulp product.

[0027] Chemical Pulping. Pulping is the process in which wood chips orother wood particulate matter is converted to fibrous form. Chemicalpulping requires cooking of the chips in solution with a chemical, andincludes partial removal of the coloring matter such as ligninassociated with the wood. The pulp of the invention can be prepared bythe Kraft process as described herein. In general, the pulping processuseful in making the pulp of the invention is a full chemical pulpingmethod using sodium hydroxide and sodium sulfide at a pH greater thanabout 12, at a temperature from about 160 to about 180° C. for about 0.5to about 5 hours.

[0028] The cooking liquor useful in the pulping process of the inventionincludes polysulfide. Polysulfide has the formula NaS(S)_(n)SNa, wheren=4-6. The polysulfide is used in the process to mitigate alkalinepeeling (i.e., to improve the quality of the cellulose and increase pulpyield by reducing the occurrence of destructive chemical reactionsreferred to as “peeling”). In one embodiment, in the practice of themethod, a polysulfide system provides the digester with between about 4to about 9 g/L polysulfide (as sulfur). In another embodiment, apolysulfide system provides the digester with between about 5.0 to about6.5 g/L polysulfide at about 25 to 37 percent sulfidity.

[0029] In addition to polysulfide, the pulping process of the inventionalso includes anthraquinone to mitigate alkaline peeling. Anthraquinoneis added to the polysulfide in the polysulfide system (seepolysulfide/anthraquinone processor 80 in FIG. 1). Anthraquinone is apulping additive useful to increase delignification, decreasecarbohydrate degradation, and improve pulp yield. Anthraquinone achievesthese advantageous properties through a chemical cycle that leads to thelignin reduction and the oxidation of cellulose's reducing endgroupaldehyde to a carboxylic acid. Anthraquinone is typically used in anamount up to about 0.10 percent by weight based on the total weight ofwood. In one embodiment, anthraquinone is used from about 0.03 to about0.07 percent by weight based on the total weight of wood. Modifiedanthraquinones, such as 1,4-dihydro-9,10,-dihydroxyanthracene (DDA, orsoluble anthraquinone, SAQ), can also be used.

[0030] Lignin content is typically measured during chemical pulping tomonitor the degree of cooking (i.e., delignification) or to measureresidual lignin before bleaching and between bleaching stages to monitorthe process. As noted above, lignin can be measured indirectly bymeasuring the amount of oxidant consumed by lignin in the sample.

[0031] After digesting, the pulp has a kappa number in the range fromabout 20 to about 65. In one embodiment, the kappa number afterdigesting is about 34 to 45. In one embodiment, the kappa number afterdigesting is about 36 to 43. In a further embodiment, the kappa numberafter digesting is about 38.

[0032] The products from the pulping or digesting are black liquor andbrownstock. Black liquor is the waste liquor from the pulping processand contains the original cooking inorganic elements and the degraded,dissolved wood substances. Black liquor contains lignin and ligninbyproducts and is separated from the brownstock and directed to arecovery system that separates the lignin and byproducts from theresidue cooking chemicals, which can be recycled back to the digester.After separating from black liquor, the pulp produced from the digestionis washed to provide brownstock. Screening may occur before or afteroxygen delignification. In the process of the invention, brownstock isintroduced into the oxygen delignification system.

[0033] Oxygen Delignification. The oxygen delignification system usefulin producing the pulp of the invention achieves from about 40 to about50 percent delignification.

[0034] Oxygen delignification is the delignification of pulp usingoxygen under pressure in aqueous alkali. Pressures typically range fromabout 550 to about 700 kpa (about 80 to about 120 psi). The alkali istypically sodium hydroxide present in about 3 to about 5 percent byweight based on the weight of pulp. Delignification can be carried outat temperatures from about 90 to about 130° C. for a period of time fromabout 20 to about 60 minutes. The oxygen stage can be carried out in asingle vessel or in a two-vessel system. The oxygen delignification stepmay include the use of magnesium ion in an amount from about 0.05 toabout 0.25 percent by weight based on the weight of pulp to mitigateextensive carbohydrate degradation.

[0035] After oxygen delignification, the pulp has a kappa number in therange from about 8 to about 35. In one embodiment, the kappa numberafter oxygen delignification is about 18 to 22.

[0036] Further details of oxygen delignification are described in U.S.Pat. No. 4,172,006, Method for Delignifying Wood Chips With Oxygen ByAdding Cooking Liquor Under Pressure; U.S. Pat. No. 4,295,925, TreatingPulp With Oxygen; U.S. Pat. No. 4,295,926, Method and Apparatus forTreating Pulp With Oxygen; and U.S. Pat. No. 4,298,426, Method andApparatus for Treating Pulp With Oxygen in a Multi-Stage BleachingSequence; each incorporated herein by reference in its entirety.

[0037] The pulp produced by the oxygen delignification step isintroduced to the bleaching stage.

[0038] Bleaching. Bleaching is the treatment of pulp fibers withchemical agents to increase their brightness. Brightness is a term usedto describe the whiteness of pulp or paper on a relative scale.Bleaching of chemical pulps is achieved by lignin and color removal.

[0039] Chemical pulp bleaching can be accomplished using variouschlorine- and/or oxygen-containing compounds (e.g., chlorine dioxide) incombination with alkali (e.g., sodium hydroxide) extractions in severalstages. Bleaching of chemical pulps tend to involve the use of chemicalsthat are more specific to lignin removal than to carbohydratedegradation. Each stage includes a bleaching step and a washing step toprovide progressively bleached pulp. Each stage includes a pump and amixer to mix the pulp with a bleaching chemical, a retention tower toallow time sufficient for the pulp and bleaching chemical to react, anda washer to remove the bleaching chemical and solubilized pulpcomponents from the bleached pulp. The bleaching chemical andsolubilized pulp components are typically washed from the pulp witheither fresh “mill water” or filtrate from another bleach stage in acounter current wash flow to save the use of fresh water. The wash watermay be alkaline or acid depending on the stage being washed.

[0040] In one embodiment of the method, the bleaching process is anElemental Chlorine Free (ECF) bleaching process. In the process,chlorine dioxide is the bleaching agent and aqueous sodium hydroxide isthe alkali extractant. In one embodiment, the bleaching method includesthree stages. Examples of representative stages include: D(Eop)D;D(Eo)D; DED; D(EP)D; D(Eop)P; D(Eo)P; D(EP)P; D(Eop)DED; D(Eo)DED;D(EP)DED; D(EP)D(EP)D; or D(PaP)(Eop)DEDP, among others. For thesestages, D refers to chlorine dioxide bleaching, E refers to extraction,Eo refers to extraction including oxygen, P refers to bleaching withhydrogen peroxide, Eop refers to alkaline extraction including oxygenand peroxide, and PaP refers to Papricycle.

[0041] In one embodiment of the method, the bleaching sequence includesthe following: (1) chlorine dioxide treatment, (2) sodium hydroxidetreatment with oxygen and peroxide, (3) chlorine dioxide treatment, (4)sodium hydroxide treatment, and (5) chlorine dioxide treatment.

[0042] In another embodiment of the method, the bleaching process is aTotal Chlorine Free (TCF) bleaching process (i.e., a process that doesnot include chlorine).

[0043] There are many other bleaching sequences that can be used in themethod of the invention. Listings of these sequences may be found in thestandard texts. Other stages include bleaching with chlorine, chlorinedioxide, hydrogen peroxide, or a hypochlorite. These stages areinterspersed with alkali extraction and/or washing.

[0044] After bleaching, the pulp is directed to a headbox in which thebleached pulp is slurried, deposited onto a forming wire (e.g.,fourdrinier forming wire), and dried to provide a web of bleached pulp.

[0045] Further details of pulping and bleaching devices, processes, andprocedures are described in U.S. Pat. No. 4,172,006, Method forDelignifying Wood Chips With Oxygen By Adding Cooking Liquor UnderPressure; U.S. Pat. No. 4,295,925, Treating Pulp With Oxygen; U.S. Pat.No. 4,295,926, Method and Apparatus for Treating Pulp With Oxygen; andU.S. Pat. No. 4,298,426, Method and Apparatus for Treating Pulp WithOxygen in a Multi-Stage Bleaching Sequence.

Pulp Products

[0046] The pulp of the invention can be advantageously incorporated intoabsorbent products such as tissue and towel products. Tissue and towelproducts typically include a combination of fibers.

[0047] A representative tissue product can include from about 10 toabout 40 percent by weight of the pulp of the invention. The tissue canfurther include other pulp, for example, bleached northern or southernsoftwood Kraft pulp, bleached northern or southern hardwood Kraft pulp,and chemi-thermomechanical pulp (CTMP), among others.

[0048] A representative towel product can include from about 30 to about75 percent by weight of the pulp of the invention. The towel can furtherinclude other pulp, for example, bleached northern or southern softwoodKraft pulp and chemi-thermomechanical pulp (CTMP), among others.

[0049] The pulp of the invention can also be used in making saturatingbase paper in which the base paper that includes the pulp of theinvention is saturated with a material such as, for example, melamine orlatex.

[0050] The pulp of the invention can also be advantageously incorporatedinto flexible packaging paper grades and other technical grade papers.

[0051] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for making achemical pulp, comprising: (a) pulping a lignocellulosic material incaustic sulfide to provide a first brownstock, wherein the causticsulfide comprises polysulfide and anthraquinone; (b) treating the firstbrownstock with oxygen to provide a second brownstock; and (c) bleachingthe second brownstock to provide a bleached pulp.
 2. The method of claim1, wherein the polysulfide is present in an amount from about 4 to about9 g/L in the white liquor.
 3. The method of claim 1, wherein theanthraquinone is present in an amount from about 0.03 to about 0.1percent by weight based on the weight of lignocellulosic material. 4.The method of claim 1, wherein the kappa number after pulping is in therange from about 20 to about
 65. 5. The method of claim 1, wherein thekappa number after pulping is in the range from about 36 to about
 43. 6.The method of claim 1, wherein treating the first brownstock with oxygencomprises treating with oxygen at a pressure between about 80 and about120 psi and at a temperature from about 90 to about 130° C.
 7. Themethod of claim 1, wherein the kappa number after treating with oxygenis in the range from about 8 to about
 35. 8. The method of claim 1,wherein bleaching comprises bleaching without elemental chlorine.
 9. Themethod of claim 1, wherein bleaching comprises bleaching with chlorinedioxide.
 10. The method of claim 1, wherein bleaching comprises at leasttwo chlorine dioxide bleaching stages.
 11. The method of claim 1,wherein bleaching comprises an alkali extraction stage.
 12. The methodof claim 1, wherein bleaching comprises bleaching without chlorine. 13.The method of claim 1, wherein bleached pulp has a hemicellulose contentgreater than about 17 weight percent as measured by the 18% causticsolubility test (TAPPI T-235 CM-00).
 14. A method for making a chemicalpulp, comprising: (a) pulping a lignocellulosic material in causticsulfide to provide a first brownstock, wherein the kappa number afterpulping is in the range from about 20 to about 65; (b) treating thefirst brownstock with oxygen to provide a second brownstock; and (c)bleaching the second brownstock to provide a bleached pulp.
 15. A methodfor making a chemical pulp, comprising: (a) pulping a lignocellulosicmaterial in caustic sulfide to provide a first brownstock; (b) treatingthe first brownstock with oxygen to provide a second brownstock; and (c)bleaching the second brownstock to provide a bleached pulp, whereinbleached pulp has a hemicellulose content greater than about 17 weightpercent as measured by the 18% caustic solubility test (TAPPI T-235CM-00).